RFR: bot: add counters to BotRunner

[Erik Helin]

Hi all,

please review this patch that adds a few counters to the bot runner. I've added counters for:

- exception being thrown by a work item
- a work item being scheduled to be executed
- a work item being discarded due to a more recent version of the item being enqueued for execution

All counters have the bot's name as a label to make it easy to view counters for individual bots (for example number of exceptions thrown per bot).

Testing:
- [x] Tested locally on Linux x64 using a local Prometheus instance and a local build of the bots

Thanks,
Erik

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Commit messages:
 - bot: add counters to BotRunner

Changes: https://git.openjdk.java.net/skara/pull/1153/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1153&range=00
  Stats: 11 lines in 1 file changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1153.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1153/head:pull/1153

PR: https://git.openjdk.java.net/skara/pull/1153